[#25881] DocDB: Make ysql_enable_packed_row external AutoFlag

Summary:
`ysql_enable_packed_row` was an NewInstalOnly AutoFlag and incorrectly made to a `LocalPersisted` AutoFlag in d8acbed/D41634
This changes switches it to the correct `External` class.

Fixes #25881
Jira: DB-15188

Test Plan: Jenkins

Reviewers: rthallam

Reviewed By: rthallam

Subscribers: ybase, yql

Differential Revision: https://phorge.dev.yugabyte.com/D41692

src/yb/docdb/pgsql_operation.cc

@@ -111,7 +111,7 @@ DEFINE_test_flag(int32, slowdown_pgsql_aggregate_read_ms, 0,
 
 // Disable packed row by default in debug builds.
 constexpr bool kYsqlEnablePackedRowTargetVal = !yb::kIsDebug;
-DEFINE_RUNTIME_AUTO_bool(ysql_enable_packed_row, kLocalPersisted,
+DEFINE_RUNTIME_AUTO_bool(ysql_enable_packed_row, kExternal,
                          !kYsqlEnablePackedRowTargetVal, kYsqlEnablePackedRowTargetVal,
                          "Whether packed row is enabled for YSQL.");